When the mega spells hit and their wild unstable magic leaked into the yawning cracks of the earth, some unfortunate changeling colonies were forever twisted by the taint. Over the decades, the taint and ambient radiation had mutated them from their semi-equine forms, causing them to take on more of the extremes of their insectoid features. These new changelings, known to some on the surface as "taintlings" lurk in dark tunnels carved away by caustic spittle, a natural weapon with which less fortunate delvers had also met a grisly, melting fate. Though they have long lost their ability to shapechange like their changeling ancestors, their carapaces can shift color and subtly shift form to blend in with terrain both within the cavernous underworld below and the bleak wastelands above.
This creature isn't from FoE canon or from any particular side story. I wondered what may have become of changelings that have become exposed to taint from the mega spells, and this is what I came up with.